Ohmic contact stacks employing a pure Al layer are commonly used as ohmic contacts to the surface of a III-Nitride or GaN based semiconductor device. Such contact structures are described, for example, in:                Microstructure of Ti—Al Ohmic contacts for n-Al GaN, Applied Physicals Letters 73, 2582 (1998), and        Low Resistance Si—Ti—Al—Ni—Au Multilayer Ohmic Contact to Undoped AlGaN—GaN, Electrochemical and Solid-State Letters, 7 (4) G72-G74 (2004).        
It is desired to have a very low contact resistance. Contact resistance (Rc) is often described by its Specific Contact Resistivity, (SCR) in units of ohm-cm2; or by its Specific Linear Contact Resistivity (SLCR) in units of ohm-mm. SCR and SLCR are nominally independent of the geometry of the contact. The total contact resistance of a given contact may be expressed as:
1. Rc=SCR/contact area, or
2. Rc=SLCR/Contact Width, where contact width is the direction perpendicular to current flow. SCR and SLCR are often measured by special test structures using methods known as the Transmission Line Method (TLM) or Van de Pauw Method (VDP).
Typical ohmic contacts to AlGaN employ metal stacks of pure films such as Ti/Al/Ni/Au or Ti/Al/Ti/TiW. (Conventionally film stacks are listed in the sequence in which they are deposited so that the first metal film listed is the first to be deposited on the semiconductor surface.)
The electrical resistance of the metal to semiconductor contact is a critical process and design parameter for achieving a low Rdson (on-state resistance) and high current-carrying capability of a transistor of other device being formed. This is especially true of III-Nitride semiconductors where the specific contact resistance SCR to an AlGaN surface can be 10s to 100s times that in silicon technology.
Thus, III-Nitride contact resistance depends on complex and, often, poorly understood interactions between the metalization layers, the composition of the semiconductor surface, the semiconductor surface properties and treatments, and annealing conditions.